The correctly reported difference of $16.4215$ and $6.01$ will have significant figures equal to

Difference of report $=16.4215-6.01=10.4115$

The answer for subtraction should be reported to the number of decimal places as is there in the number with minimum decimal places.

$6.01$ has the least number of decimal places.

So, the reported difference $=10.41$ ,  it has four significant figures.

Therefore, option B is correct.